MDI (a mixture of diphenylmethylene diisocyanate and polymethylene polyphenyl isocyanate) is one of the main raw materials in the polyurethane industry. The MDI reacts with polyether polyalcohol or polyester polyalcohol, in the presence of catalyst, foaming agent and the like, to produce various polyurethane polymer materials which are used widely in the fields of polyurethane rigid- and semi-rigid foams plastics, of polyurethane products by reaction injection molding- or reinforced reaction injection molding process, as well as in the fields of heat insulation materials, synthetic fibers (urethane elastic fiber), binders and elastomers, etc.
It is well-known in the polyurethane industry that a condensation reaction of aniline and formaldehyde is conducted in the presence of a hydrochloric acid catalyst to obtain firstly polymethylene-polyphenyl-polyamine, and a phosgenation reaction of polymethylene-polyphenyl-polyamine is then conducted to produce monomeric MDI and polymeric MDI.
The condensation reaction of aniline and formaldehyde in the presence of a hydrochloric acid catalyst for obtaining polymethylene-polyphenyl-polyamine is one of the key steps in the MDI production process. If the formaldehyde and aniline can not be mixed at the molecular level in a short time, it may result in local excess of formaldehyde, thus by-products and net-like high polymers will be produced and the quality of the targeted polyamine will be affected. In addition, since the reaction mixture has a high viscosity and the raw materials flow in a laminar pattern, the mixing effect deteriorates accordingly, usually being accompanied with local overheating. Therefore, the by-products increase and the product quality fluctuates; in the worst situation, the pipes may be blocked and the reaction apparatus has to be shut down. The mixed state of formaldehyde with other raw materials is the key factor of the whole condensation reaction, in order to disperse formaldehyde in the reaction system rapidly and uniformly and avoid local excess. Up to now, the mixing/reaction apparatuses used in the production process are the mixing pump, the jet mixer, the dynamic mixer or the static mixer, etc.
U.S. Pat. No. 6,720,455B2 describes two condensation reaction modes, one of which generally adopts a process of firstly reacting the aniline with hydrochloric acid to produce aniline hydrochloride, then mixing aniline hydrochloride and formaldehyde in a proper mixing mode and conducting a pre-condensation reaction by controlling the reaction temperature in between 30-80° C. and controlling the residence time of the reaction materials in the reactor, and subsequently raising the temperature of reaction materials to 100-160° C. by heating the condensation solution in a stepwise or continuous mode to conduct a molecular rearrangement reaction.
Another reaction mode adopts the process of firstly mixing the aniline and formaldehyde to make them react with each other by controlling the reaction temperature in between 60-85° C., then removing the water from the above reaction solution with phase separation or other process, such as rectification, and mixing the obtained intermediate products and hydrochloric acid catalyst with a proper mixing mode to conduct a condensation reaction by controlling the temperature at 30-80° C. After the reaction was completed, the temperature was controlled at 100-160° C. in a stepwise or continuous temperature rising mode to conduct a molecular rearrangement reaction. However, this prior patent does not disclose the specific mode or apparatus which was used to mix the raw materials.
U.S. Pat. No. 6,673,970B1 describes a condensation process which can decrease or reduce impurities in DAM which may have impact on the color of the final MDI products, that is to say, the more the impurities is contained in DAM, the darker the color of MDI products is. In the process, the first step is to form aniline hydrochloride, the second step is to sufficiently form an intermediate aminobenzylaniline by adding the reactant formaldehyde gradually for controlling the reaction process; the third step is, in the molecular rearrangement process, to add about 70%-80% of stoichiometric amount of base to partially neutralize the reaction mixture.
U.S. Pat. No. 3,517,062 presents a process flow of continuous polyamines preparation. Aniline and hydrochloric acid form a salt in a CSTR (Continuous Stirred-Tank Reactor), which is introduced into another CSTR in which formaldehyde is fed continuously and the condensation reaction is conducted, then the reaction mixture is introduced into a tubular reactor to conduct the molecular rearrangement reaction.
U.S. Pat. No. 3,260,751 describes a continuous condensation process, in which the proposed flow condition of fluid is Re=4500-100000 to ensure a turbulent fluid condition, so that the formation of high molecular polymer may be inhibited and thus the blocking of apparatus may be avoided. If the materials can not be mixed sufficiently, the ratios of compositions of the raw materials may vary with local positions, and the desired product can not be obtained with satisfaction. Therefore, this patent proposes a L-type mixing device which consists of two stainless steel tubes having diameter of ¼ inch. The aniline hydrochloride passes through the L-type device and is introduced tangentially into a formaldehyde feeding tube from the bottom of the L-type. The diameter of aperture is 1/32 inch and a jacket casing is provided outside the tube for cooling. If several nozzles are distributed on the tubular reactor to feed the materials in batch, a formation of turbulent flow can also be obtained.
The main objects of these methods all relate to reduce the contents of impurities in polymethylene polyphenyl polyamines, so as to improve their performance in use of the MDI products and, at the same time, to inhibit formation of substance having high molecular weight caused by non-uniform distribution of formaldehyde, and so as to avoid the deposition and blocking phenomenon on the inner walls of pipes and apparatus. However, it is impossible for these known methods to attain a fast and sufficient mixing of aniline and formaldehyde, especially when used in a large-scale continuous production process, the above-mentioned problems may well occur.
High gravity technology is a novel technique which strengthens the mass transfer, heat transfer and the micro-mixing processes using a much higher gravity (referred to as “high gravity”) than the earth gravity, and which is carried out by producing a simulative high gravity condition on earth through rotation. This technique can greatly improve the conversion rate and selectivity in reactions, significantly reduce the volume of reactors, simplify the process and flow pattern, obtain high efficiency and low power consumption, and reduce the pollutants discharge. As indicated in some studies and analyses, under a high gravity condition, the molecular diffusion between molecules of different sizes and the inter-phase mass transfer processes are much faster than that in a normal gravity field. When gas-liquid, liquid-liquid or liquid-solid two-phases flow towards each other and come into contact in a porous medium under a high gravity which is hundreds or even a thousand times higher than the earth gravity, an enormous shear force breaks the liquid into liquid films, liquid threads and liquid droplets of nano-scale and thus generate enormous and quick-renewed phase interfaces, so that the inter-phase mass transfer rate is improved by 1-3 order of magnitude than that the conventional tower devices could offer, and the micro-mixing and mass transfer process is improved greatly. Such kind of high gravity rotating bed device has been disclosed in the prior art. On the above basis, studies on practical application of high gravity technology have gained some important progresses and its application has been extended from the physical process, such as separation and desorption, into chemical reaction processes. However, up to now there is no report concerning the application of high gravity technology in condensation reaction for preparing polymethylene polyphenyl polyamines.